Chapter 5: Loops in Java
Introduction to Loops
Sometimes we need to repeat a set of statements. For example:
In such cases, loops are used to execute a set of statements repeatedly.
Types of Loops in Java
1. while loop
Executes statements while condition is true
2. do-while loop
Executes statements at least once, then checks condition
3. for loop
Most commonly used when number of iterations is known
4. Enhanced for loop
Used to iterate over arrays and collections
While Loop
The while loop executes a set of statements as long as the condition is true.
Syntax:
while (condition) {
// Statements to be executed
}Example 1: Print numbers 1 to 10
public class WhileExample {
public static void main(String[] args) {
int i = 1;
while (i <= 10) {
System.out.println(i);
i++; // Important: increment to avoid infinite loop
}
}
}Example 2: Sum of first n natural numbers
import java.util.Scanner;
public class SumNatural {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.print("Enter n: ");
int n = scanner.nextInt();
int sum = 0;
int i = 1;
while (i <= n) {
sum += i;
i++;
}
System.out.println("Sum = " + sum);
scanner.close();
}
}Important: Always ensure the loop variable is updated inside the loop to avoid infinite loops!
Do-While Loop
The do-while loop executes the statements at least once, then checks the condition.
This is useful when you want to execute the loop body at least once regardless of the condition.
Syntax:
do {
// Statements to be executed
} while (condition);Example: Menu-driven program
import java.util.Scanner;
public class MenuExample {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int choice;
do {
System.out.println("\n--- Menu ---");
System.out.println("1. Add");
System.out.println("2. Subtract");
System.out.println("3. Exit");
System.out.print("Enter your choice: ");
choice = scanner.nextInt();
switch (choice) {
case 1:
System.out.println("Addition selected");
break;
case 2:
System.out.println("Subtraction selected");
break;
case 3:
System.out.println("Goodbye!");
break;
default:
System.out.println("Invalid choice!");
}
} while (choice != 3);
scanner.close();
}
}While vs Do-While
While Loop
Do-While Loop
For Loop
The for loop is used when the number of iterations is known beforehand.
It's the most commonly used loop in programming.
Syntax:
for (initialization; condition; increment/decrement) {
// Statements to be executed
}For Loop Components:
1. Initialization
Executed once at the beginning
int i = 0
2. Condition
Checked before each iteration
i < 10
3. Update
Executed after each iteration
i++
Example 1: Print numbers 1 to 10
public class ForExample {
public static void main(String[] args) {
for (int i = 1; i <= 10; i++) {
System.out.println(i);
}
}
}Example 2: Print multiplication table
import java.util.Scanner;
public class MultiplicationTable {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.print("Enter a number: ");
int num = scanner.nextInt();
System.out.println("Multiplication table of " + num + ":");
for (int i = 1; i <= 10; i++) {
System.out.println(num + " x " + i + " = " + (num * i));
}
scanner.close();
}
}Enhanced For Loop (For-Each)
The enhanced for loop is used to iterate over arrays and collections.
It's also known as the "for-each" loop and provides a cleaner syntax.
Syntax:
for (dataType variable : array/collection) {
// Statements to be executed
}Example: Iterate over an array
public class EnhancedForExample {
public static void main(String[] args) {
int[] numbers = {1, 2, 3, 4, 5};
// Enhanced for loop
System.out.println("Enhanced for loop:");
for (int num : numbers) {
System.out.println(num);
}
}
}Note: Enhanced for loop is read-only. You cannot modify the array elements using this loop.
Loop Control Statements
Loop control statements change the execution flow of loops.
1. break Statement
Terminates the loop immediately
for (int i = 1; i <= 10; i++) {
if (i == 5) {
break; // Exit loop when i equals 5
}
System.out.println(i);
}
// Output: 1, 2, 3, 42. continue Statement
Skips the current iteration and continues with the next
for (int i = 1; i <= 10; i++) {
if (i == 5) {
continue; // Skip when i equals 5
}
System.out.println(i);
}
// Output: 1, 2, 3, 4, 6, 7, 8, 9, 10Nested Loops
A loop inside another loop is called a nested loop.
Nested loops are useful for working with 2D arrays, patterns, and complex iterations.
Example 1: Print a pattern
public class StarPattern {
public static void main(String[] args) {
int rows = 5;
for (int i = 1; i <= rows; i++) {
for (int j = 1; j <= i; j++) {
System.out.print("* ");
}
System.out.println();
}
}
}
// Output:
// *
// * *
// * * *
// * * * *
// * * * * *Example 2: Number pattern
public class NumberPattern {
public static void main(String[] args) {
int rows = 4;
for (int i = 1; i <= rows; i++) {
for (int j = 1; j <= i; j++) {
System.out.print(j + " ");
}
System.out.println();
}
}
}
// Output:
// 1
// 1 2
// 1 2 3
// 1 2 3 4Practice Set
Quick Quiz 1:
What will be the output of this code?
for (int i = 0; i < 5; i++) {
if (i == 3) {
continue;
}
System.out.print(i + " ");
}Answer: 0 1 2 4 (3 is skipped due to continue)
Quick Quiz 2:
How many times will "Hello" be printed?
for (int i = 1; i <= 3; i++) {
for (int j = 1; j <= 2; j++) {
System.out.println("Hello");
}
}Answer: 6 times (3 × 2 = 6)
Chapter 5 - Practice Set
1. Pattern Printing
Write a program to print the following pattern:
**** *** ** *
2. Sum of Even Numbers
Write a program to sum first n even numbers using a while loop.
3. Multiplication Table
Write a program to print multiplication table of 10 in reverse order.
4. Factorial
Write a program to find factorial of a given number using for loops.
5. Prime Numbers
Write a program to print all prime numbers between 1 and 50.
6. Fibonacci Series
Write a program to print first n terms of Fibonacci series.